Clarify how to check for EOF in SDL_ReadIO() documentation.

Fixes https://github.com/libsdl-org/SDL/issues/8564

include/SDL3/SDL_iostream.h

@@ -481,9 +481,7 @@ extern SDL_DECLSPEC Sint64 SDLCALL SDL_TellIO(SDL_IOStream *context);
  *
  * This function reads up `size` bytes from the data source to the area
  * pointed at by `ptr`. This function may read less bytes than requested. It
- * will return zero when the data stream is completely read, or on error. To
- * determine if there was an error or all data was read, call
- * SDL_GetIOStatus().
+ * will return zero when the data stream is completely read, and SDL_GetIOStatus() will return SDL_IO_STATUS_EOF, or on error, and SDL_GetIOStatus() will return SDL_IO_STATUS_ERROR.
  *
  * \param context a pointer to an SDL_IOStream structure.
  * \param ptr a pointer to a buffer to read data into.